Conventional loudspeakers use a piston movement at the centre of a diaphragm to cause air to vibrate to produce sound waves. The outer rim of the diaphragm is supported by a frame and the driven centre of the diaphragm is supported by a damper. The diaphragm is usually cone-shape to provide stiffness in its direction of vibration.
In contrast, in a flat panel, panel form or panel loudspeaker, vibrations are applied to specific points on a flat-panel diaphragm by actuators to generate bending waves in the diaphragm. In this way, multiple point sound sources are provided across the entire diaphragm as bending waves distributed over the diaphragm across a range of frequencies in random phases. Panel loudspeakers or panel form loudspeakers are generally described in U.S. Pat. No. 6,332,029 and European patent application with publication No. EP0847661.
Distributed mode (or DM) loudspeakers (or DMLs) are flat panel loudspeakers in which sound is produced by inducing uniformly distributed vibration modes in the panel. A mode is a predictable standing-wave—bending pattern that is obtained by stimulating the panel with a single spot frequency. It is dependent on the physical constraints of the panel and the frequency. DMLs are available in a variety of forms, including as part of a larger structure with rigid boundaries such as described in U.S. Pat. No. 6,546,106 and European patent application with publication No. EP1068770, or as a display element in an electronic device such as described in U.S. Pat. No. 7,174,025 and European patent application with publication No. EP1084592.
While it is common for a DML to be driven by actuators whose size is small compared with the panel, that is not necessarily the case. U.S. Pat. No. 6,795,561 and European patent application with publication No. EP1197120 described activation by an electrically active planar actuator of size similar to the panel being driven.
There is demand for thin electronic devices with audio capability and many of the existing DML applications are considered too thick for these applications. From a technical perspective, large-area electrically active planar actuators are considered attractive for such applications. However, these large-area patches are unattractive due to high component costs, low efficiency and a poor acoustic response.
Furthermore, for providing audio capability with a display, with the introduction of organic light emitting diode (OLED) displays, small patches can be used behind the display and the small patches are no longer restricted to the localised edge drive of the panels as has been the case with backlit liquid crystal displays (LCDs). As a consequence, a method is sought of using a plurality of small patches or arrays of patches, which are cheap, and do not overly stiffen the substrate.
Each actuator is controlled by an electrical input and a panel loudspeaker controlled by n actuators has n input channels (where n is an integer and n>1). From, for example, Audio Engineering Society Convention Paper 5611 presented at the 112th Convention 10-13 May 2013, Munich, Germany, “Multichannel Inverse Filtering of Multiexciter Distributed Mode Loudspeakers for Wave Field Synthesis” Etienne Corteel, Ulrich Horbach and Renato S. Pellegrini, it is known to attempt to calibrate the response of an n channel panel loudspeaker by individually applying an impulse to each input individually and observing the impulse response from each input individually. This calibration is then used on the fly during use of the panel loudspeaker to control the actuators of the panel loudspeaker. This is computationally expensive.